1. Field of the Invention
The invention relates to a method for controlling the weighting of a data signal in the at least two antenna elements of a multi-element transceiver of a telecommunications network, which data signal is to be transmitted by at least one weighting vector from said transceiver to a terminal. The invention equally relates to a method for controlling the weighting of a data signal in the antenna elements of a first and at least a second transceiver of a telecommunications network, when a terminal is in soft handover with the first and at least the second transceiver. The invention moreover relates to such a transceiver and a module in such a transceiver, to such a terminal and a module in such a terminal and such a telecommunications network.
2. Description of the Prior Art
For telecommunications systems, in particular for systems using WCDMA (wideband code division multiple access), it is known to use base stations with several transmit antenna elements in order to be able to provide for transmission diversity. The antenna elements are controlled with complex coefficient weight vectors, each weight vector forming a beam in a certain direction, and where each weight vector transmits a set of modulated signals. Corresponding user equipment requires at least one antenna for reception, which also extracts the statistical characteristics of the channel using a pilot signal or a training signal transmitted from the base station or from any other multi-element transmitter or transceiver.
In order to control the weighting of two antenna elements of a transceiver effectively, that is by taking into account changing transmission paths to the terminal, it has been proposed in the WCDMA specification: “3GPP RAN WG1, Physical Layer-General Description”, v. 2.0.0, April 1999, to exploit short-term channel fluctuations estimated in the terminal and fed back to the transceiver. Proceeding from this specification, the document by Ari Hottinen, Olav Tirkkonen, Risto Wichman, Nokia Research Center: “Closed-Loop Transmit Diversity Techniques for Multi-Element Transceivers” discloses different concepts for using short term feedback information for transceivers with more than two antenna elements.
In one alternative, with the assumption of correlated spatial channels and a specific parameterized weight set for the antenna array with M elements, a particular parameterized beam-forming concept is used in which the transmit weight/array vector, parameterized by θ is given by:w(θ)=[1, e(j2πd sin(θ))/λ, . . . , e(j2π(M−1)d sin(θ))/λ]T/√{square root over (M)}, where d is the distance between the elements in the array.For example, with a Uniform Linear Array (ULA) d=λ/2 is set, where λ is the carrier wavelength. The feedback can be calculated for example using the eigenvector corresponding to the largest eigenvalue of the channel matrix HHH, where H=(h1, . . . , hM) and where hm is the impulse response vector between the m array element and the terminal. When denoting this eigenvector by emax and solving
            θ      *        =          arg      ⁢                        max          θ                ⁢                                                                                          w                  ⁡                                      (                    θ                    )                                                  H                            ⁢                              e                max                                                          2                      ,the phase at the transmit element m is wm=e(j2□(m−1)d sin(□)/□). Then, the same relative phase, calculated using common channel measurements, is used between all neighboring transmitting elements. Thus, in this example only one coefficient has to be signalled to the network regardless of the number of transmitting elements. It is not necessary here for the terminal to know precisely the antenna structure of the transceiver, as it is in this example sufficient only to know (and signal information related to) the relative phase difference between antenna elements. It is clear that other array structures have different parameterizations. Alternatively the terminal can transmit simply the eigenvector coefficients and let the transceiver quantize the received eigenvector to the best matching parameterized array manifold. In the ULA (Uniform Linear Array) case this manifold is represented by w(O) above.
It is mentioned in the document that in the presence of high Doppler frequencies (e.g. at velocities above 50 km/h) the feedback modes show diminishing returns, or even performance degradation when compared to open-loop concepts (including single antenna transmission). The performance degradation at high speeds is in part due to signalling inaccuracies and in part due to exacerbated channel estimation problems in closed-loop modes. Even though it is indicated that different techniques can be applied to improve the performance in higher Doppler frequencies, long term spatial channel properties are not dealt with. Accordingly, the proposed methods are only beneficial with slowly time varying channels or when the control is sufficiently accurate within the channel time coherence. Moreover, the structural properties related to the downlink channel are not taken into account.
The document “Advanced closed loop Tx diversity concept (eigenbeamformer)”, 3GPP TSG RAN WG 1, TSGR1#14(00)0853 Meeting #14, Jul. 4-7, 2000, Oulu, Finland, by Siemens describes a possibility of taking into account the long term variations as well.
This document is aimed at three channel classes that are to benefit from the proposed method. The first class includes spatially uncorrelated channels. A second class included spatially coherent channels which are frequency non-selective. The third class, which is considered as most important class, includes spatially coherent channels which are frequency selective or spatially partially correlated channels. In this class, the received signal deteriorates when using only short term feedback information, if the terminal exceeds the velocity threshold imposed by the coherence time and the feedback bandwidth. Accordingly, the velocity threshold for the terminal is to be increased.
To this end, the dominant eigenbeams are calculated in the terminal by estimating long term spatial signal covariance matrices from received vectors of spatial channel estimation of the n-th temporal tap and carrying out an eigenanalysis on those matrices. Each resulting eigenvector with a complex number for each antenna element constitutes an eigenbeam. The dominant eigenbeams are fed back to the transceiver. A feedback rate of 1500 bps is proposed for the downlink eigenbeamformer. The long term information bits for the eigenbeams and the short term information bits for eigenbeam selection are multiplexed over 15 slots as illustrated in three examples in FIGS. 1a, 1b and 1c. 
Several problems are either not addressed in this document or the proposed solutions are unsatisfactory when considering many crucial aspects required by a working solution. As an example, the method proposed for signalling the long term channel information to the transceiver is unsatisfactory. In addition, the required signalling reliabilities for the long term channel and the short term channel vectors are not addressed, and the joint efficient use of downlink channel structures in the presence of common and dedicated pilot channels are neglected.